Acquisition and tracking of a spreading signal often require a period of time to integrate the received signal from the output of a match filter. If carrier phase is estimated, it is called a coherent integration. If the carrier phase information is ignored, it is called a non-coherent integration.
A long integration period is especially needed to maintain an acceptable signal-to-noise ratio (SNR) after integration when the received signal is weak. However, long integration period usually causes two problems in communication system. One problem is the update rate and loop bandwidth of phase lock loop and delay lock loop are limited. Another problem is the detection rate of the receiver is slowed down when a blocking or jamming condition happens during the integration, which causes longer reacquisition time. The coherent and non-coherent integrations are schematically shown in FIG. 1. As known in this field, an input spreading signal is de-spreading by a mixer 1 with a code replica provided by a code generator 2. I and Q components of the signal are Doppler-wiped-off by mixers 3, 4 with a carrier provided by a carrier generator 5. The processed I and Q components are integrated by integration and dump units 6 and 7 and stored in a coherent memory 9 as coherent integrations. The coherent integrations can be further accumulated in a non-coherent memory 9 as non-coherent integrations.
Coherent integration is usually used as a method of acquisition and tracking for a spreading signal under normal signal strength receiving condition. Non-coherent integration is especially useful when trying to increase dispreading gain at weak signal strength receiving condition, for example, conditions of signal path blocking, jamming or indoor.
For GPS L1 CA code acquisition, coherent integration interval is usually less than 20 ms if bit synchronization is not exactly known. At signal tracking stage or pull-in stage, the coherent integration length, TC, can be extended to 20 ms after bit sync is found. In practical case, a non-coherent integration interval, which means N non-coherent integration number multiply by TC ms of coherent integration time, could be 2 seconds or even longer, for tracking or acquiring a weak GPS signal. A reference flow chart of the a conventional receiver channel operation is shown in FIG. 2,
In conventional receiver design, if a receiver loses lock for a weak spreading signal during a first interval of non-coherent integration, to re-acquire the spreading signal, a second interval of non-coherent integration is required. All information of the first non-coherent integration interval is lost at the beginning of second interval of non-coherent integration.
As mentioned, to successfully acquire the weak spreading signal, a non-coherent integration of a sufficiently long interval without deeper blocking or jamming is required. That is, the integration of the weak spreading signal needs to be executed uninterruptedly, which means no sudden block or jamming, in original signal path for the sufficiently long interval. However, if deeper signal blocking or jamming occurs during the integration interval, and it fails to pass detection threshold after integration, another long period is likely to be required by the convention method.
Example is shown in FIG. 3. As shown in the drawing, deeper signal blocking or jamming occurs in the first non-coherent integration interval and persists across the second non-coherent integration interval. Therefore, as in this example, a qualified integration result (which means signal-to-noise ratio (SNR) is greater than threshold) cannot be obtained until the third integration interval is finished. In other words, it consumes three intervals to obtain a qualified integration result, which means to pass SNR detection threshold after integration. This could be a significantly long period of time for satellite signal tracking or re-acquisition. Assuming non-coherent integration interval is 2 sec, for example, a short period of blocking or jamming during integration is likely to make a receiver channel lose lock to satellite for 6 sec.
In search stage, if the deeper blocking or jamming happens on hypothesis of the right code phase and carrier frequency, more time is required for searching other hypotheses. The time length depends on total signal search range. Such a problem often happens in conventional spreading signal receiver designs. Therefore, there is a need for an improved method to shorten the time periods required for acquiring and tracking the spreading signal.
Present invention provides a solution to overcome the drawback described above.